Method for confining a dense plasma



Feb. 4, 1964 A. J. HATCH METHOD FOR CONFINING A DENSE PLASMA 2 Sheets-Sheet 1 Filed Jan. 9, 1962 United States Patent 3,120,477 METHOD FOR CONFINING A DENSE PLASMA Albert J. Hatch, Chicago, Ill., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Jan. 9, 1962, Ser. No. 165,264 3 Claims. (Cl. 176-1) The present invention relates to methods for containing and compressing plasma discharges and more specifically to the use of radio frequency electromagnetic waves to contain and compress plasma discharges which may produce thermonuclear-derived neutrons and ultimately useful power.

It is appreciated in the art that thermonuclear power is a possible source of almost unlimited energy. Further, it has been determined that thermonuclear reactions are most likely to occur in high density plasmas in which the kinetic energy of the particles making up the plasma are at a very high energy level. It is necessary that the plasma be contained at a requisite density and that the particles be excited to a proper temperature with respect to the density with either a continuous or pulsed type of operation.

One of the major problems that has been apparent since the conception of obtaining a thermonuclear reaction is the confinement of a plasma at a sufficiently high density and temperature for a long enough period of time to enable a thermonuclear reaction to take place. One suggested method is by the use of rotating radiofrequency fields as described in Ulrich et al., application S.N. 833,894, filed August 14, 1959, now Patent 3,022,- 236, granted February 20, 1962. Ulrich et al. show the use of rotating radiofrequency fields excited within a spherical cavity in the TE mode to generate and confine a plasma therein.

The present invention describes methods for confining a dense plasma by the use of stationary radiofrequency fields.

It is one object of the present invention to provide a method for confining and compressing a plasma discharge.

It is another object of the present invention to provide a method for producing a stabilized plasma discharge.

It is another object of the present invention to provide a method for containing and compressing a plasma discharge by means of stationary radio frequency fields.

It is still another object of the present invention to provide a method for containing and compressing a plasma discharge which may ultimately lead to the production of useful power by means of nuclear reactions.

Other objects of the present invention will become apparent as the detailed description proceeds.

In general the present invention comprises evacuating a cavity and exciting electromagnetic waves therein in a magnetic quadrupole mode. A tritium-deuterium gas is introduced into the cavity where the electric and magnetic fields existing therein ionize the gas and confine and compress it about the center thereof.

More complete understanding of the invention will best be obtained from consideration of the accompanying drawings in which:

FIGURE 1 shows a cross section of the field configuration of the TE magnetic quadrupole mode in a spherical cavity with a central conducting plasma.

FIGURE 2 shows the potential function of a point diamagnetic and dielcctrophoretic sphere in a nonuniform radiofrequency field which function has a gradient proportional to the time average force acting on the point sphere.

FIGURE 3 is a vertical section view of a device for the practice of this invention with some of its components shown schematically.

FIGURE 4 is an enlarged sectional view taken along lines 4 -4 in FIGURE 3, showing the details of an electromagnetic field coupling loop.

FIGURE 5 is an enlarged, fragmentary section view of the portion encircled and referenced 5 in FIGURE 3, showing the details of construction of the vacuum manifold surrounding the housing.

FIGURE 6 is a cross section of a spherical cavity showing the electric and magnetic fields of the TE mode and a gas inlet introduced therein.

For the purposes of this invention, a dense plasma is defined as one in which the ratio of mp to w is greater than unity. wp is the plasma frequency squared and is equal to n being equal to the plasma density, -e being equal to the charge on an electron, m being equal to the mass of an electron, and 6 being equal to the permittivity of free space. to is the radian frequency of the electromagnetic fields within a cavity.

Figure 1 shows a cross sectional of the electric and magnetic fields within a spherical cavity 10 which is excited in the T13 mode. The dots and crosses 12 represent the electric field lines and the broken lines 14 represent the magnetic field lines.

When a dense plasma is exposed to nonuniform electric and magnetic fields as shown in FIGURE 1, the force F, exerted thereon by electric fields 12 is equal to VKEQEZ (K being a constant, V being a gradient operator, and 6 being the permittivity of free space), and the force F exerted thereon by the magnetic fields 14 is equal to Ku ,H (K being a constant, V being a gradient operator, and n being the permeability of free space). The net average effect F of the electric and magnetic fields 12 and 14 is equal to This force phenomenologically is due to the dielectrophoretic and diamagnetic behavior of the plasma in the non-homogeneous fields. The quantity K (V: u H E E is the net potential exerted by the electric and magnetic fields 12 and 14 and may be represented by the character Thus F 'y.

In FIGURE 2, values of 7 are shown for the magnetic quadrupole mode in the cavity 10 of radius R of FIGURE 1. The radial variations of 7 have been plotted on an arbitrary scale at colatitudes of 10 degrees. The plotted lines 18 of radial variations of 'y are defined as potentials of a point diamagnetic and dielectrophoretic plasma sphere in a nonuniform radiofrequency field whose gradient is proportional to the time average force (F acting on the point sphere. In each curve 18, 'y is zero at the center of the cavity 10 where there is a field node, becomes positive for small values of r/R, and reaches a crest 20 in the vicinity of 0.2 r/R 0.43 as shown. The ratio of the maximum to minimum height of the crest 20 is approximately 9:1 and is proportional to H Therefore the ratio of the maximum to minimum magnetic fields is approximately 3:1.

The crest 2t! defines the boundaries of a potential well 22 which will exist as shown within the cavity 10 in FIG- URE 1, since the crest 20 of FIGURE 2 is symetrical in all four quadrants of the cavity 10. The volume of the potential well 22 within the cavity 10 is approximately 2.6% of the cavity volume. The apparent well 24 in the vicinity of r/R=0.6 in FIGURE 2 occurs at an E field maximum rather than at a field node and is not suitable for containment of dense plasmas since it means negative pressure on the plasma. Therefore it is apparent that the potential well 22 is the only well wherein a dense plasma may be confined and compressed. The well 22 defines the boundaries; the actual shape of a plasma contained in the well 22 will have the general shape of a hypocycloid of revolution.

It has been previously established (see Ulrich et 21]., application S.N. 833,894, filed August 14, 1959) that the minimal requirements which may produce neutrons by a nuclear reaction in a plasma is a plasma temperature of 1 Rev. at a plasma density of 2x10 particles per meter and a confining force acting on the plasma interface of approximately 420 gauss.

Consideration is now given to an actual device for containing and compressing a plasma discharge in a stable manner by means of stationary radio-frequency fields.

FIGURE 3 illustrates such a radiofrequency machine 26 comprising a metal sphere 28 defining a spherical cavity 30 in which the plasma discharge is confined and compressed by radiofrcquency fields. The metal sphere 28 is strong enough to withstand the hard vacuum applied =to the cavity 30 and is constructed of two hemispheres of A" thick steel, copper clad on the interior thereof.

For operation at a density of 2x10 per meter and a temperature of l kev., the power requirement to produce 420 gauss at the plasma interface in the cavity 30 when operated in the TE mode is approximately 3 megawatts. With the formation and compression of a plasma in the cavity 30, the resonant frequency of the cavity 30 will increase by l to 1.5% within a time period of 100 microseconds. This percentage change in resonant frequency is due to the net amount of volume change in the magnetic field displaced by the plasma. Since the place of confinement is about a field node and is in a potential well, the percentage displaced thereby will below. However, even though the percentage change 'in the resonant frequency of cavity 30 is only 1 to 1 /z%, since it is desired to use an operating frequency for excitation of 735 megacycles, the R-F generator system must therefore have broadband characteristics.

To produce the required power at the proper excitation frequency in the cavity 30, a single water-cooled, ceramic envelope, super power triode 32 is used. The tube 32 should be capable of 5 megawatts output at approximately 700 megacycles. An RCA A2346 triode has these characteristics and is used in the device of FIGURE 3.

Coupling loop 34 transmits energy from tube 32 to the evacuated cavity 30 through a ceramic dome 36. FIG- URE 4 illustrates the construction of the ceramic dome 36 with the coupling loop 34 therein. The dome 36 is fabricated of a strong ceramic such as pyroceram and is welded to a metal ring 38 attached to the outside of the steel sphere 28.

The super power triode 32 is adapted to operate as a selfcxcited, grid driven oscillator and is connected so that it follows any resonant frequency change that occurs during the formation and subsequent confinement of the plasma. The cavity 30, when empty resonates at 735 megacycles in only the T mode, and when a fully developed plasma core exists, resonates at 742 megacycles. The R-F generating system may be made to follow this frequency shift by minimizing the stored energy in the external circuit elements and making the main cavity an integral part of the feed-back path for the oscillator 32. By connecting the grid 40 of the triode 32 through an amplifier and phase shift network 42 to a coupling loop 44 penetrating into the cavity 30, the frequency changes caused by compression of the plasma will be fed back to the tube 32 adjusting the frequency output thereof accordingly.

The machine 26 as described may be used in a single pulse or a repetitive pulse operation. The output of the 4 tube 32 may be pulsed by pulsing means within the plate supply 46 furnishing power to the tube 32. The machine 26 may be also operated as a steady state machine by eliminating the pulsing means within the plate supply 46, providing a tube capable of megawatt operation on a steady basis is used.

A high quality vacuum system to insure adequate control of gas purity is an essential of the machine 26. A critical feature of this system is the design of the cavity pumping parts. These parts must have adequate evacuating conductance, but should not appreciably affect the cavity Q nor contribute to the excitation of spurious RF modes. A study of the current density vectors at the surface of a cavity for the electromagnetic excitation of the type used in this device will show that the vector loci of the currents are perpendicular to the meridian plane of the cavity and are zero along the equatorial plane thereof. The vector loci of these currents increase with increasing distance away from the equatorial plane of the cavity, reach a maximum at about 45, and then de crease till they are again zero at the poles of the cavity. A large number of small slots 48 elongated in the direction of the current vector will thus provide a minimum of distortion of the electromagnetic field modes. In the machine 26 of FIGURE 3, 30 slots, 4 cm. long and 1 cm. high are provided. The slots 48 provide communication between the cavity 30 and the vacuum manifold 50.

The vacuum manifold 50 comprises a flanged annular L-cross sectioned member 52 welded to each separable hemispherical portion of the sphere 28. The flanges 54, on annular members 52, are held together by bolts 56 (FIGURE 5) which thereby serve to secure the two separable hemispheres together. Three gold O-rings 58, 58a, 58b, of increasing major diameters are positioned between the confronting flanges 54 to seal the interior of the metal sphere 28. An air outlet line 60 is connected to the space between the two innermost O-rings 58 and 58a, and leads to a mechanical pump (not shown) to reduce any leakage therethrough.

The main pumping system comprises a diffusion pump 62, connected through an appropriate oil vapor battle 64 and shutoff valve 66 to the vacuum manifold 50. Any standard oil difiusion pump may be used in the pumping system which will operate to evacuate the interior of the sphere 28 at a pumping rate of approximately 50 liters/ second. Other types of vacuum pump may be used to establish the vacuum within sphere 28. However, the pumping system must be capable of attaining a vacuum within sphere 28 of approximately 10- mm. of Hg.

A gas inlet 66 connected to the manifold 50 supplies the gas to be ionized into the interior of the sphere 28. The gas mixture used in the embodiment of FIGURE 3 comprises approximately 50% by volume of each of the gases tritium and deuterium and is introduced at a base pressure of approximately 10* to 10- mm. of Hg. A vacuum pressure gauge 68 connected to the manifold 50 by a gas line 70 measures the pressure with the sphere 28.

To dissipate heat in the walls of sphere 28, cooling coils 72 surround the sphere 28. Water or other suitable cooling means is circulated through the coils 72 by a pump 74 which passes the heated water to a conventional heat exchanger 76. The heat transferred in the heat exchanger may be passed to the atmosphere or converted to electrical power through well known means.

Referring now to the operation of the machine 26 of FIGURE 5, the cavity 30 of sphere 28 is pumped out to a pressure of approximately 10- mm. of Hg. The system is then preferably outgassed by halting the entire system in an oven at a temperature of approximately 400 C.

The gaseous mixture of tritium and deuterium is first introduced into the cavity 30 via gas inlet 66. Voltage is then applied to the triode 32 to establish the R-F field in the cavity 39 in the T13 mode at a frequency of 735 megacycles. The introduced gas will be ionized by the maximum electric fields existing in the magnetic quadrupole mode as hereinbefore described. The ionized particles, created by the maximum electric fields, will diffuse away from these fields under the influence thereof. Some of the ionized particles will collect in the potential Well of the magnetic quadrupole and when the density thereof becomes critical, i.e., greater than one, the ionized plasma will become subject to the forces of the magnetic fields and will be compressed in the potential Well of the magnetic quadrupole. A magnetic confining force of approximately 420 gauss will be established at the plasma interface in the hypocycloid thereof. and will compress the plasma to a volume of 3,000 cc.

An alternate method of operation comprises first establishing the R-F magnetic quadrupole mode and then introducing the deuterium-tritium gas. However, with this method, the gas should be injected Within the potential well of the magnetic quadrupole; otherwise the gas, if injected by gas inlet 66, will form a plasma within the apparent well 24 of FIGURE 2. FIGURE 6 shows the position of the gas inlet 66' with respect to the magnetic quadrupole mode when the machine of FIGURE 3 is operated in this alternate mode. The gas inlet 66' of FIGURE 6 is comprised of a material such as quartz.

The principal design features of the machine hereinbefore described are as follows:

Cavity diameter cm 75 Empty cavity frequency, TE

mode mc 735 Loaded cavity frequency mc 742 Empty cavity Q -100,000

(Q being a figure of merit 21r maximum energy stored at resonance) energy lost per cycle R-F pulse length milliseconds 1-10 R-F pulse power mw 3 Cavity filling time microseconds -25 Cavity ultimate pressure mm. Hg l0 Operating base pressure mrn.Hg to 10- Persons skilled in the art will, of course, readily adapt the teachings of the invention to methods and embodiments far different than the methods and embodiments herein described. Accordingly, the scope of the protection atforded the invention should not be limited to the particular embodiments and methods descirbed and shown above but shall be determined only in accordance with the appended claims.

What is claimed is:

1. A method of confining and compressing a plasma discharge comprising evacuating a radiofreqnency cavity, introducing at a low pressure a low Z gas into said cavity, and exciting at a particular frequency stationary electromagnetic wavcs in only the TE magnetic quadrupole mode within said cavity, said electromagnetic waves ionizing said low Z gas and confining and compressing the plasma therefrom at the center of said acvity.

2. A method of confining and compressing a plasma discharge comprising evacuating a radiofrequency cavity, exciting at a particular frequency stationary electromagnetic waves in only the TE magnetic quadrupole mode within said cavity, and introducing at a low pressure a low Z gas into said cavity, said electromagnetic waves ionizing said low Z gas and confining and compressing the plasma therefrom at the center of said cavity.

3. A method of confining and compressing a plasma discharge comprising evacuating a radiofrequency cavity, exciting at a particular frequency stationary electromagnetic waves in the TE magnetic quadrupole mode within said cavity, introducing at a low pressure a low Z gas into said cavity about the center thereof, said electro magnetic waves ionizing said low Z gas and confining and compressing the plasma therefrom at the center of said cavity.

References Cited in the file of this patent UNITED STATES PATENTS Ulrich et al Feb. 20, 1962 OTHER REFERENCES 

1. A METHOD OF CONFINING AND COMPRESSING A PLASMA DISCHARGE COMPRISING EVACUATING A RADIOFREQUENCY CAVITY, INTRODUCING AT A LOW PRESSURE A LOW Z GAS INTO SAID CAVITY, AND EXCITING AT A PARTICULAR FREQUENCY STATIONARY ELECTROMAGNETIC WAVES IN ONLY THE TE210 MAGNETIC QUADRUPOLE MODE WITHIN SAID CAVITY, SAID ELECTROMAGNETIC WAVES IONIZING SAID LOW Z GAS AND CONFINING AND COMPRESSING THE PLASMA THEREFROM AT THE CENTER OF SAID CAVITY. 